KLF4 and CD55 expression and function depend on each other.

The transcription factor Kruppel-like factor 4 (KLF4) regulates the expression of immunosuppressive and anti-thrombotic proteins. Despite its importance in maintaining homeostasis, the signals that control its expression and the mechanism of its transactivation remain unclarified. CD55 [aka decay accelerating factor (DAF)], now known to be a regulator of T and B cell responses, biases between pro- and anti-inflammatory processes by controlling autocrine C3a and C5a receptor (C3ar1/C5ar1) signaling in cells.

Disabled C3ar1/C5ar1 Signaling in Foxp3+ T Regulatory Cells Leads to TSDR Demethylation and Long-Term Stability.

Demethylation of the T regulatory cell (Treg)-specific demethylation region (TSDR) of the Foxp3 gene is the hallmark of Foxp3+ Treg stability, but the cellular signaling that programs this epigenetic state remains undefined. In this article, we show that suppressed C3a and C5a receptor (C3ar1/C5ar1) signaling in murine Tregs plays an obligate role. Murine C3ar1-/-C5ar1-/- Foxp3+ cells showed increased suppressor of cytokine signaling 1/2/3 expression, vitamin C stabilization, and ten-eleven translocation (TET) 1, TET2, and TET3 expression, all of which are linked to Treg stability.

Vascular Endothelial Cells Produce Coagulation Factors That Control Their Growth via Joint Protease-Activated Receptor and C5a Receptor 1 (CD88) Signaling.

As per the classical view of the coagulation system, it functions solely in plasma to maintain hemostasis. An experimental approach modeling vascular reconstitution was used to show that vascular endothelial cells (ECs) endogenously synthesize coagulation factors during angiogenesis. Intracellular thrombin generated from this synthesis promotes the mitotic function of vascular endothelial cell growth factor A (VEGF-A).

RTK signaling requires C3ar1/C5ar1 and IL-6R joint signaling to repress dominant PTEN, SOCS1/3 and PHLPP restraint.

How receptor tyrosine kinase (RTK) growth signaling is controlled physiologically is incompletely understood. We have previously provided evidence that the survival and mitotic activities of vascular endothelial cell growth factor receptor-2 (VEGFR2) signaling are dependent on C3a/C5a receptor (C3ar1/C5ar1) and IL-6 receptor (IL-6R)-gp130 joint signaling in a physically interactive platform. Herein, we document that the platelet derived and epidermal growth factor receptors (PDGFR and EGFR) are regulated by the same interconnection and clarify the mechanism underlying the dependence.

Follicular B2 Cell Activation and Class Switch Recombination Depend on Autocrine C3ar1/C5ar1 Signaling in B2 Cells.

The involvement of complement in B2 cell responses has been regarded as occurring strictly via complement components in plasma. In this study, we show that Ab production and class switch recombination (CSR) depend on autocrine C3a and C5a receptor (C3ar1/C5ar1) signaling in B2 cells. CD40 upregulation, IL-6 production, growth in response to BAFF or APRIL, and AID/Bcl-6 expression, as well as follicular CD4 cell CD21 production, all depended on this signal transduction.

CD55 Is Essential for CD103 Dendritic Cell Tolerogenic Responses that Protect against Autoimmunity.

Recent studies traced inflammatory bowel disease in some patients to deficiency of CD55 [decay-accelerating factor (DAF)], but the mechanism underlying the linkage remained unclear. Herein, we studied the importance of DAF in enabling processes that program tolerance in the gut and the eye, two immune-privileged sites where immunosuppressive responses are continuously elicited. Unlike oral feeding or ocular injection of ovalbumin in wild-type (WT) mice, which induced dominant immune tolerance, identical treatment of DAF mice or DAF to WT bone marrow chimeras did not.

VEGFR2 survival and mitotic signaling depends on joint activation of associated C3ar1/C5ar1 and IL-6R-gp130.

Purified vascular endothelial cell (EC) growth factor receptor-2 (VEGFR2) auto-phosphorylates upon VEGF-A occupation , arguing that VEGR2 confers its mitotic and viability signaling in and of itself. Herein, we show that, in ECs, VEGFR2 function requires concurrent C3a/C5a receptor (C3ar1/C5ar1) and IL-6 receptor (IL-6R)-gp130 co-signaling. C3ar1/C5ar1 or IL-6R blockade totally abolished VEGFR2 auto-phosphorylation, downstream Src, ERK, AKT, mTOR and STAT3 activation, and EC cell cycle entry.

Anaphylatoxin Receptors C3aR and C5aR1 Are Important Factors That Influence the Impact of Ethanol on the Adipose Secretome.

Chronic ethanol exposure results in inflammation in adipose tissue; this response is associated with activation of complement as well as the development of alcohol-related liver disease (ALD). Adipose communicates with other organs, including liver, via the release of soluble mediators, such as adipokines and cytokines, characterized as the "adipose secretome." Here we investigated the role of the anaphaylatoxin receptors C3aR and C5aR1 in the development of adipose tissue inflammation and regulation of the adipose secretome in murine ALD (mALD).

C5aR1 regulates migration of suppressive myeloid cells required for costimulatory blockade-induced murine allograft survival.

Costimulatory blockade-induced murine cardiac allograft survival requires intragraft accumulation of CD11b Ly6C Ly6G regulatory myeloid cells (Mregs) that expand regulatory T cells (Tregs) and suppress effector T cells (Teffs). We previously showed that C5a receptor (C5aR1) signaling on T cells activates Teffs and inhibits Tregs, but whether and/or how C5aR1 affects Mregs required for transplant survival is unknown. Although BALB/c hearts survived >60 days in anti-CD154 (MR1)-treated or cytotoxic T-lymphocyte associated protein 4 (CTLA4)-Ig-treated wild-type (WT) recipients, they were rejected at ~30 days in MR1-treated or CTLA4-Ig-treated recipients selectively deficient in C5aR1 restricted to myeloid cells (C5ar1 xLysM-Cre).

T Cell Expression of C5a Receptor 2 Augments Murine Regulatory T Cell (T) Generation and T-Dependent Cardiac Allograft Survival.

C5aR2 (C5L2/gp77) is a seven-transmembrane spanning receptor that binds to C5a but lacks motifs essential for G protein coupling and associated signal transduction. C5aR2 is expressed on immune cells, modulates various inflammatory diseases in mice, and has been shown to facilitate murine and human regulatory T cell (T) generation in vitro. Whether and how C5aR2 impacts in vivo T generation and pathogenic T cell-dependent disease models have not been established.

DAF in diabetic patients is subject to glycation/inactivation at its active site residues.

Decay accelerating factor (DAF or CD55) is a cell associated C3 and C5 convertase regulator originally described in terms of protection of self-cells from systemic complement but now known to modulate adaptive T cell responses. It is expressed on all cell types. We investigated whether nonenzymatic glycation could impair its function and potentially be relevant to complications of diabetes mellitus and other conditions that result in nonenzymatic glycation including cancer, Alzheimer's disease, and aging.

TLR-Induced Murine Dendritic Cell (DC) Activation Requires DC-Intrinsic Complement.

Induction of proinflammatory T cell immunity is augmented by innate dendritic cell (DC) maturation commonly initiated by TLR signaling. We demonstrate that ligation of TLR3, TLR4, and TLR9 induces murine DC production of complement components and local production of the anaphylatoxin C5a. In vitro, ex vivo, and in vivo analyses show that TLR-induced DC maturation, as assessed by surface phenotype, expression profiling by gene array, and functional ability to stimulate T cell responses, requires autocrine C3a receptor and C5a receptor (C3ar1/C5ar1) signaling.

Differential contribution of complement receptor C5aR in myeloid and non-myeloid cells in chronic ethanol-induced liver injury in mice.

Background: Complement is implicated in the development of alcoholic liver disease. C3 and C5 contribute to ethanol-induced liver injury; however, the role of C5a receptor (C5aR) on myeloid and non-myeloid cells to progression of injury is not known.

Methods: C57BL/6 (WT), global C5aR-/-, myeloid-specific C5aR-/-, and non-myeloid-specific C5aR-/- mice were fed a Lieber-DeCarli diet (32%kcal EtOH) for 25 days.

Shear stress-dependent downregulation of the adhesion-G protein-coupled receptor CD97 on circulating leukocytes upon contact with its ligand CD55.

Adhesion G protein-coupled receptors (aGPCRs) are two-subunit molecules, consisting of an adhesive extracellular α subunit that couples noncovalently to a seven-transmembrane β subunit. The cooperation between the two subunits and the effect of endogenous ligands on the functioning of aGPCRs is poorly understood. In this study, we investigated the interaction between the pan-leukocyte aGPCR CD97 and its ligand CD55.

Absence of signaling into CD4⁺ cells via C3aR and C5aR enables autoinductive TGF-β1 signaling and induction of Foxp3⁺ regulatory T cells.

Signaling through the G protein-coupled receptors for the complement fragments C3a and C5a (C3aR and C5aR, respectively) by dendritic cells and CD4(+) cells provides costimulatory and survival signals to effector T cells. Here we found that when signals from C3aR and C5aR were not transduced into CD4(+) cells, signaling via the kinases PI(3)Kγ, Akt and mTOR ceased, activation of the kinase PKA increased, autoinductive signaling by transforming growth factor-β1 (TGF-β1) initiated and CD4(+) T cells became Foxp3(+) induced regulatory T cells (iT(reg) cells). Endogenous TGF-β1 suppressed signaling through C3aR and C5aR by preventing the production of C3a and C5a and upregulating C5L2, an alternative receptor for C5a.

Antigen-presenting cell-derived complement modulates graft-versus-host disease.

Acute graft-versus-host disease (GvHD) is a serious complication of allogeneic hematopoietic cell transplantation (allo-HCT) that results from donor allogeneic T cell attack on host tissues. Based on previous work implicating immune cell-derived C3a and C5a as regulators of T cell immunity, we examined the effects of locally produced C3a and C5a on murine T cell-mediated GvHD. We found that total body irradiation, a conditioning regimen required to permit engraftment of allo-HCT, caused upregulation and activation of alternative pathway complement components by recipient APCs.

A novel role for CD55 in granulocyte homeostasis and anti-bacterial host defense.

Background: In addition to its complement-regulating activity, CD55 is a ligand of the adhesion class G protein-coupled receptor CD97; however, the relevance of this interaction has remained elusive. We previously showed that mice lacking a functional CD97 gene have increased numbers of granulocytes.

Methodology/results: Here, we demonstrate that CD55-deficient mice display a comparable phenotype with about two-fold more circulating granulocytes in the blood stream, the marginated pool, and the spleen.

Complement regulates CD4 T-cell help to CD8 T cells required for murine allograft rejection.

Although induction of CD8 T-cell responses to transplants requires CD4-cell help, how this help is transmitted remains incompletely characterized. In vitro, cognate interactions between CD4 T cells and dendritic cells (DCs) induce C3a and C5a production. CD8(+) T cells lacking C3a receptor (C3aR) and C5a receptor (C5aR) proliferate weakly to allogeneic DCs despite CD4 help, indicating that CD4-cell help is mediated, in part, through DC-derived C3a/C5a acting on CD8(+) T cell-expressed C3aR/C5aR.

Interaction of decay-accelerating factor with echovirus 7.

Echovirus 7 (EV7) belongs to the Enterovirus genus within the family Picornaviridae. Many picornaviruses use IgG-like receptors that bind in the viral canyon and are required to initiate viral uncoating during infection. However, in addition, some of the enteroviruses use an alternative or additional receptor that binds outside the canyon.

Immune cell-derived c3 is required for autoimmune diabetes induced by multiple low doses of streptozotocin.

Objective: The complement system contributes to autoimmune injury, but its involvement in promoting the development of autoimmune diabetes is unknown. In this study, our goal was to ascertain the role of complement C3 in autoimmune diabetes.

Research Design And Methods: Susceptibility to diabetes development after multiple low-dose streptozotocin treatment in wild-type (WT) and C3-deficient mice was analyzed.

The intrinsic complement regulator decay-accelerating factor modulates the biological response to vascular injury.

Objective: To investigate whether the presence of decay-accelerating factor (or CD55), an intrinsic complement regulator, protects against the development of vascular disease, given that complement activation can affect leukocytes and platelets.

Methods And Results: Leukocyte-platelet complexes are critical for the initiation and progression of atherosclerosis and restenosis; however, the mechanism by which these processes promote vascular injury is incompletely defined. We performed femoral artery wire injury in Daf1(-/-) mice and their wild-type controls.

Deletion of either CD55 or CD97 ameliorates arthritis in mouse models.

Objective: CD55 (decay-accelerating factor) is best known for its role in the negative regulation of the complement system. Indeed, lack of this molecule leads to disease aggravation in many autoimmune disease models. However, CD55 is abundantly present on fibroblast-like synoviocytes and is also a ligand of the adhesion-class heptahelical receptor CD97, which is expressed by infiltrating macrophages.

Decay accelerating factor is essential for successful corneal engraftment.

In contrast to immune restrictions that pertain for solid organ transplants, the tolerogenic milieu of the eye permits successful corneal transplantation without systemic immunosuppression, even across a fully MHC disparate barrier. Here we show that recipient and donor expression of decay accelerating factor (DAF or CD55), a cell surface C3/C5 convertase regulator recently shown to modulate T-cell responses, is essential to sustain successful corneal engraftment. Whereas wild-type (WT) corneas transplanted into multiple minor histocompatibility antigen (mH), or HY disparate WT recipients were accepted, DAF's absence on either the donor cornea or in the recipient bed induced rapid rejection.

Primed CD8(+) T-cell responses to allogeneic endothelial cells are controlled by local complement activation.

CD8 T cells primed by transplantation recognize allogeneic class I MHC molecules expressed on graft vascular endothelium and contribute to allograft injury. We previously showed that immune cell-derived complement activation fragments are integral to T cell activation/expansion. Herein we tested the impact of local complement production/activation on T cell/endothelial cell (EC) interactions.